The invention relates to cosmetic and pharmaceutical compositions. More particularly, the invention relates to water-in-oil compositions having enhanced clarity.
In recent years, the availability of crystal clear personal care and cosmetic products has become increasingly important to the consumer. A clear product is perceived as light, clean, fresh and often cooling, features that can be important in certain types of products, for example, products for use in the sun, or makeup or skin care products to be used in warmer weather. Products of this type have largely been limited to very specific forms, however, such as gels, soap-based sticks, and thin liquids; the more traditional cosmetic vehicles, i.e., water and oil emulsions, have been less successfully converted to a clear form. Water-in-oil products are particularly desirable for certain uses, because they spread easily and evenly, forming a long-lasting, continuous, protective film on the skin, and also mimic the skin""s natural lipid barrier. Such characteristics are important in products that are desirably water-proof and/or long-wearing, e.g., such as sunscreens, moisturizers, and self-tanners. The nature of the emulsion, however, in which two mutually insoluble or nearly insoluble phases are dispersed one within the other, does not easily achieve clarity because of the discontinuous droplet phase dispersed within the continuous phase: opacity typically results due to the refraction and reflection at the emulsion interface, as light is transmitted through the continuous phase and into the dispersed phase.
Transparent emulsions can be prepared in the form of microemulsions, in which the dispersed phase is essentially solubilized in the continuous phase by the action of a surfactant, and often, a cosurfactant. Rather than discrete isolated dispersed phase droplets, micelles comprising an inner core of the disperse phase material surrounded by a layer of surfactant are formed. These systems upon formation are normally clear, forming very small micellar droplets, but ordinarily require fairly large quantities of surfactant. Normally, it is desirable to keep surfactant levels low in cosmetic compositions, however, because they can strip the skin of its natural lipid barrier, and thus leave the skin more susceptible to irritants.
Alternate efforts to resolve this problem have turned to matching of refractive indices between the components of the two phases. Water and most water-soluble materials have a relatively low refractive index, i.e., about 1.3-1.4, whereas oils and oil soluble materials have refractive indices that range from about 1.4-1.5 for silicone oils, up to about 1.5-1.6 for hydrocarbons. In order to match the two phases, it is necessary to add a further component to the mix to either adjust the water phase index up, or reduce the oil phase index. Most frequently, the additional component is a glycol, usually in fairly high percentages, added to the water phase. Although the resulting product will appear to be clear, it is normally undesirable to use high levels of glycols in cosmetic products, since these compounds are themselves often irritating to the skin of the user. Additionally, since the amount of water that can stably be incorporated into a water-in-oil emulsion is limited, the use of large quantities of glycols in the aqueous phase necessarily reduces the amount of water, and the amount of water soluble actives, that can be used in the emulsion.
There continues to be a need for a clear water-in-oil emulsion that permits reduction in the amount of irritating surfactants and glycols, and in which the amount of water and water-soluble actives can be correspondingly increased.
The present invention relates to cosmetic and pharmaceutical clear water-in-oil emulsions, comprising a water droplet phase dispersed in an oil phase, the average droplet size less than about 1xcexc, the refractive indices of the oil and water phases being matched to within 3 parts in 10,000. In a preferred embodiment, the composition comprises less than about 15% by weight of a glycol, and also comprises no more than 1.5% of a surfactant. Preferably the composition contains at least about 30% of water.
The invention also relates to a method for making a clear water-in-oil emulsion comprising adjusting the refractive indices of the oil and water phases to within 3 parts in 10,000, employing no more than about 15% of a glycol, and emulsifying the phases under high shear to obtain an average dispersed water droplet size of less than about 1xcexc.
The compositions of the invention are true water-in-oil emulsions, rather than microemulsions, with exceptional clarity, a trait previously difficult to achieve in a true emulsion without the use of substantial quantities of irritating substances such as glycols and surfactants. The clarity of the emulsion is obtained by the control of two different aspects of the composition. The first aspect is matching the refractive indices of the oil and water phases. In the present case, the refractive index of the water phase is adjusted by the addition of an a water-soluble material other than a glycol. By xe2x80x9cglycolxe2x80x9d in the context of the present specification and claims is meant a C2-4 diol such as ethylene, butylene or propylene glycol. In a preferred embodiment, the added compound used for adjustment is a glycerin polymer. The polymer can contain from 2-60 glycerin subunits; preferably, the polymer contains from 2-10 glycerin subunits, and most preferably contains from 2-5 glycerin subunits. Examples of useful polymers include diglycerol, triglycerol, oligoglycerols, and polyglycerols, or a combinations thereof. Particularly preferred is diglycerol.
The amount of glycerin polymer to be added to the aqueous phase will vary depending upon the refractive index of the oil phase to be matched. The refractive index of the water phase and the oil phase should be matched to within 0.0003 refractive index units. The absolute value of the refractive index is not critical, provided that both are matched to within the specified limits or better. It is possible to create a clear emulsion using any oil, or oil-soluble material, as a component of the oil phase. The major oil component can be either hydrocarbon, silicone-based oils, oily esters or any combination of these. Examples of oils useful in the invention include, but are not limited to, vegetable oils, such as coconut oil, jojoba oil, corn oil, olive oil, sunflower oil, palm oil, and soybean oil; carboxylic acid esters such as isostearyl neopentanoate, cetyl octanoate, cetyl ricinoleate, octyl palmitate, dioctyl malate, Icoco-dicaprylate/caprate, decyl isostearate, myristyl myristate; animal oils such as lanolin and lanolin derivatives, tallow, mink oil or cholesterol; glyceryl esters, such as glyceryl stearate, glyceryl dioleate, glyceryl distearate, glyceryl linoleate, glyceryl myristate; nonvolatile hydrocarbons, such as isoparaffins, squalane, or petrolatum; and volatile hydrocarbons, such as straight or branched chain hydrocarbons having from 8-20 carbon atoms, e.g., decane, dodecane, tridecane, tetradecane, and C8-20 isoparaffins. However, because of the refractive index of silicone oils is closer to that of water, it is often preferred to use one or more silicone oils as the principle oil component. The silicone oil can be selected from volatile silicones, such as the cyclic (cyclomethicone) or linear (dimethicone) polydimethylsiloxanes, having from three to nine carbon atoms. The silicone oil may also be a nonvolatile oil, such as higher molecular weight dimethicones, dimethiconol, phenyl trimethicone, methicone, or simethicone. Typically, the oil component will comprise from about 10 to about 50%, preferably about 20 to about 30%, by weight of the composition as a whole, and the water component from about 30 to about 90 %, preferably about 50 to about 90%, most preferably about 70-80%. As noted above, the amount of refractive index-adjusting component employed in the water phase will vary depending on the identity of the oil to be matched, as well as the other components to be included in the composition; however, the amount will ordinarily be in the range of about 0.01 to about 10%, preferably about 0.5 to about 5%.
The composition of the invention is unusual in its use of relatively small amounts of glycols. While some glycols may be used in the water phase to assist in matching refractive indices, preferably no more than about 15%, more preferably no more than about 5%, of standard glycols are employed. If used, the glycol is preferably selected from the group consisting of butylene glycol, propylene glycol, ethylene glycol and trimethylene glycol, dipropylene glycol, isoprene glycols and polyethylene glycols. Because of the refractive index adjustment can be achieved with the use of relatively low levels of glycols and glycerin polymers, it is possible for the composition to contain up to 90% water and water-soluble actives, exclusive of glycols and glycerin polymers.
The composition also achieves a clear water-in-oil emulsion without the formation of a microemulsion, and consequently, with the use of very little surfactant. When used, the amount of surfactant is ordinarily less than about 3%, preferably less than about 1.5%. The preferred surfactant for use in the composition is a low HLB (i.e., no higher than 5) surfactant, examples of which can readily be obtained in McCutcheon""s Emulsifiers and Detergents, North American Edition, 2000.
In addition to the actual components of the composition, the processing of the composition is important in achieving a superior clarity. It is desirable to achieve a water phase average droplet size of less than 1xcexc, preferably in the range of from about 100 to about 900 nm. This is typically achieved by processing the components in a high shear homogenizer, which will reduce droplet size beyond the size capable of being obtained with standard mechanical mixing. Examples of appropriate homogenizers are Silverson homogenizers (L4RT-A; Silverson Machines, Ltd., Chesham, England) and Greerco homogenizers(IL; Baldor Electric Co., Ft. Smith, AK.). Processing speeds in excess of 3000 rpm are generally needed in order to achieve the preferred droplet size. The process should be preferably conducted without heat, more preferably at room temperature. In order to facilitate obtaining the desired small size droplet, and a stable product, it is preferable to utilize a low HLB emulsifier. The preferred emulsifier has an HLB value of no greater than 6,preferably from about 2 to about 4 and preferably also has a low refractive index. Suitable water-in-oil emulsifiers include, but are not limited to, sorbitan derivatives such as sorbitan laurate and sorbitan palmitate; alkoxylated alcohols such as laureth-4; hydroxylated derivatives of polymeric silicones, such as dimethicone copolyol; alkylated derivatives of hydroxylated polymeric silicones, such as cetyl dimethicone copolyol; glyceryl esters such as polyglyceryl-4 isostearate; beeswax derivatives such as sodium isostearoyl-2-lactylate; lecithin; and mixtures thereof. In conjunction with the preferred oil component being a silicone oil, the preferred emulsifiers are hydroxylated derivatives of polymeric silicones and alkylated derivatives thereof. Particularly preferred is dimethicone copolyol.
The compositions of the invention exhibit exceptional clarity, preferably with an optical clarity of less than 100 NTU, more preferably with a clarity of 50 NTU or less, can be used as a vehicle for a variety of purposes, in fact, for any type of product, either cosmetic or pharmaceutical, in which a water-in-oil vehicle is appropriate. In this regard, the basic components of the clear emulsion, as described above, will frequently be combined with other cosmetic and pharmaceutical components. Examples of such components include, but are not limited to antioxidants, antimicrobials, sunscreens, analgesics, anesthetics, anti-acne agents, antidandruff agents, antidermatitis agents, antipruritic agents, anti-inflammatory agents, antihyperkeratolytic agents, anti-dry skin agents, antiperspirants, antipsoriatic agents, antiseborrheic agents, hair conditioners and hair treatment agents, antiaging agents, antiwrinkle agents, antihistamine agents, skin lightening agents, depigmenting agents, wound-healing agents, vitamins, corticosteroids, self-tanning agents, or hormones. The composition can also include any one or more of strictly cosmetic ingredients, such as pigments, dyes, emollients, humectants, stabilizers, and fragrances and can be employed in various types of cosmetic products, such as bath preparations, hair conditioners, cleansers, makeup removers, and the like.